This invention is directed to a simple method of forming a hydrophobic, highly oleophilic coating on an aluminum surface which prevents excessive oxidation of the surface and which is very compatible with and readily wet by oily liquids such as metal-working lubrications.
Many processes are presently available for forming protective coatings on aluminum surface, such as anodizing, plating, chemical conversion coating, painting and the like. The resultant coatings, although designed for long life, require extensive surface pretreatments and are quite expensive. However, frequently, only short-term protection is needed or desired, for example, to prevent the formation of water stain or other oxidation products in the shipping or storing of semifabricated aluminum products, such as coiled sheet and the like. The application of various types of oils to continuous sheet prior to coiling has usually been found to be inadequate to effectively prevent water stain. In some instances, it is desirable to prevent the gradual buildup of natural oxide on the aluminum surface, for example, in welding applications and adhesive bonding applications because the buildup of natural oxide can interfere with these processes.
However, no simple and inexpensive process is presently known which will form a protective coating without interfering with subsequent fabrication, particularly when lubricants are applied to the surface, and without detrimentally affecting the surface appearance of the fabricated product.
A simple, inexpensive process for coating aluminum is described by Wittrock et al in U.S. Pat. No. 3,726,721, assigned to the present assignee, but the friable coating formed by this process can interfere with the surface appearance after forming or coating operations.
Kubie in U.S Pat. No. 2,963,391 describes a process for forming a coating designed as an extrusion lubricant wherein the aluminum surface is first treated with an ammonia-laden alkaline solution containing a fatty acid (or equivalent salt or ester thereof) and then baked at about 400.degree. F to form a coating having unknown properties except for lubrication.
Marosi in U.S. Pat. No. 3,849,207 describes and claims a process for treating aluminum and other metals wherein the surface is treated with an alkaline, sodium formate solution and then coated with a clear resinous film to form a sepia-colored coating. The nature of the coating formed during treatment in the alkaline sodium formate solution is not described in the reference. However, it has been found that the aluminum surface underlying such a coating is readily susceptible to water stain or other oxidation in much the same manner as untreated aluminum because the coating is readily wet and penetrated by water or aqueous solutions.
A simple, inexpensive method, which temporarily prevents extensive oxidation of an aluminum surface by water or other media and which also prepares the surface for the application of a lubricant, has been needed for many years but has been heretofore unavailable.
It is against this background that the present invention was developed.